The invention relates to a process for preparing an amino nitrile mixture comprising aminoacetonitrile (AAN) and from 5 to 70% by weight of iminodiacetonitrile (IDAN).
AAN and IDAN are important starting materials or intermediates in the preparation of ethylene amines by catalytic hydrogenation. Thus, for example, EP-A 1 209 146 relates to a process for the continuous hydrogenation of nitrites to primary amines, with ethylenediamine (EDA) being prepared from AAN as starting material and diethylenetriamine (DETA) being prepared from IDAN as starting material.
Processes for preparing AAN and IDAN are likewise known. Thus, U.S. Pat. No. 5,079,380 relates to a process for preparing AAN, in which ammonia (NH3), formaldehyde (HCHO) and hydrocyanic acid (HCN) are reacted at least 100° C. Furthermore, it is generally known that as an alternative formaldehyde and hydrocyanic acid can firstly be reacted to form formaldehyde cyanohydrin (FACH) as intermediate which subsequently reacts further with ammonia to form AAN. The molar ratio of ammonia to FACH or to formaldehyde and hydrocyanic acid is usually 4:1 [mol/mol]. The temperatures in the AAN synthesis are from 50 to 80° C. and the pH is about 10. The preparation of IDAN likewise usually starts out from ammonia and FACH or ammonia, formaldehyde and hydrocyanic acid. The reaction to form IDAN is generally carried out at higher temperatures (about 100-150° C.), a lower pH of about 5-7 and a smaller proportion of ammonia than in the corresponding synthesis of AAN. Such processes for preparing IDAN are described, for example, in EP-A 426 394 or U.S. Pat. No. 4,895,971. As an alternative, the preparation of IDAN can also be carried out by reacting urotropin (hexamethylenetetraamine; HMTA) with hydrocyanic acid and formaldehyde, as described, for example, in U.S. Pat. No. 3,988,360.
U.S. Pat. No. 2,511,487 relates to a process in which IDAN is prepared from AAN. Here, AAN is mixed with FACH in a molar ratio of about 1:0.3-1.5 [mol/mol] and heated at from 100 to 150° C. in the presence of a mineral acid stabilizer such as phosphoric acid. To obtain a very high yield of IDAN, the reaction preferably takes place at from 135 to 150° C. for a maximum of 15 minutes. However, U.S. Pat. No. 2,511,487 does not mention that the mixture of AAN and FACH is passed through a (separate) apparatus; rather, the reaction takes place in a customary flask provided with cooling facilities.
In all the above-described processes for preparing AAN or IDAN, attempts are always made to prepare the respective product (AAN or IDAN) in a very high purity. However, processes in which an amino nitrile mixture comprising mainly (as amino nitrites) AAN and IDAN in a defined composition can be prepared deliberately are not known. To prepare such an amino nitrile mixture comprising AAN and IDAN, it is in principle possible to mix the two main components AAN and IDAN in the desired ratio. However, disadvantages of this method are that, firstly, two separate materials (AAN and IDAN) have to be employed and, secondly, the isolation and handling of the IDAN which is obtained as a solid frequently presents problems.